VT automatic losing 4th gear

[p.macgregor]

I have a 97 VT Acclaim wagon with auto trans, a few weeks ago it started to lose 4th gear - no lurching gear changes, just a gradual slip from 4th to 3rd can only notice as the engine revs start climbing

The auto trans shop wants $80 to run a diagnostic so they can quote, no money back if they find something wrong and I get them to repair.....seems like an easy way to make $80 to me

Could this be as simple as a major box service to get it right again?